Use of devices including non-volatile memories is becoming ever more widespread in a variety of electronic devices. In particular, the extensive use that is being made of mass consumer electronic equipment such as cellular telephones, portable audio reproducers and digital photographic and television cameras has led to a need for satisfying a growing demand for electronic devices having low power consumption and high memory capacity. These needs have led to a gradual reduction of the size of the transistors of the integrated circuits, a consequent reduction of the supply voltages and a growth of the complexity of the circuits. In fact, the need for assuring control and correct management of the internal signals, especially the analog ones, even when the supply voltages are low, calls for supplementary circuit functions and particular measures in the design of the integrated circuit.
An often fundamental requirement of electronic devices of the aforementioned type is a brief time for accessing the non-volatile memory. The proper operation of non-volatile memories in general, and “flash” memories in particular, calls for relatively intense electric fields that cannot be obtained directly from the supply voltage of the device. In these cases use is made of voltage booster circuits. A typical voltage booster circuit comprises a charge pump, usually integrated into the device containing the memory, that assures the generation of the necessary high voltages starting from the supply voltage. Starting from a given supply voltage, it is possible to realize booster circuits capable of providing positive boosted voltages and booster circuits capable of providing negative boosted voltages to satisfy all the biasing needs of the non-volatile memories. Hereinafter the term converter will also be used to identify a voltage booster circuit.
If the shortest possible memory access time is to be obtained, the time the converter needs to generate the operating voltages has to be as brief as possible: this is equivalent to maximizing both the start-up speed, i.e. the speed at which the output voltage of the converter is brought from zero to the operating value, and the recovery speed, i.e. the speed with which the output voltage is brought back to its operating value after it has undergone a lowering (in absolute value) on account of an overload. In other words, it is important to minimize the settling time, i.e. the time necessary for the output voltage of the converter to attain its operating value and to maintain it with a given precision.
Other important characteristics of a charge pump converter are the precision and stability of the output voltage. Precision may be obtained via a regulation circuit that enables or disables the charge pump in such a manner as to maintain the output voltage at a predetermined value. There may nevertheless occur the situation in which the charge pump has to work in a variable current regime, i.e. with absorption of current by the variable load in the course of time, so that the output voltage will not be as stable as would be desirable, but subject to a ripple. If the converter and the memory are to function correctly, it is therefore desirable to dispose of a fast regulation system to make sure that the regulated output value will be obtained in a short time and yet have the desired precision.
The following discussion refers to a cell of a non-volatile memory of the “flash” type. The programming (or writing) of the cell is obtained by applying very precise voltage values in several equal and successive phases to its terminals (gate, source, drain and body). In the case of a flash cell formed by “triple well” technology, for example, the cell is biased at the beginning of each of these phases by bringing the drain electrode to a predetermined positive voltage (typically +4V), the source electrode to ground potential (0V), and the body electrode to a predetermined negative voltage (typically −1.5V); the gate electrode is initially brought to a positive voltage (typically +2.5V) that in each subsequent phase is increased by a predetermined voltage step (300 mV, for example). In the second part of each individual phase the state of the cell is examined via a verification operation: whenever the threshold voltage of the cell is not comprised in a range of predetermined values, the operation described above is repeated, thus increasing the gate voltage by another step, otherwise the programming is terminated.
The above discussion makes it clear that, given the numerous cell biasing operations that always call for non negligible settling times, the programming of a flash memory occupies a relatively long time. In the case of a memory of the multilevel type, i.e. a memory in which the cells can be programmed at several threshold levels and are therefore capable of storing more than one bit per cell, this problem is particularly strongly felt, because the number of programming phases is greater and the sum of the settling times is therefore likewise greater. In certain cases the programming time may arrive at relatively very high values, for example, as much as several hundred microseconds.
Naturally, the problem related to the settling time concerns not only the programming of the cells, but also all the other operations in which the cell electrodes and the parasitic capacitances associated with them have to be biased with voltages generated by charge pumps.